The present invention relates to the field of PVD coated tools.
In the prior art, tool steel and cemented carbide cutting tools have been coated by CVD (chemical vapor deposition) to enhance their cutting lifetime during machining operations. Typical coatings which have been applied are titanium carbide, titanium carbonitride, titanium nitride and alumina. One preferred type of CVD coating is a triphase coating in which a titanium carbide layer is bonded to the substrate, a titanium carbonitride layer is bonded to the titanium carbide layer and then a titanium nitride layer is bonded to the outside of the titanium carbonitride layer (see, for example, E. N. Smith et al U.S. Pat. No. 4,035,541 and W. Schintlmeister U.S. Pat. No. 4,101,703).
PVD (physical vapor deposition) coatings have been applied to tool steel and cemented carbide cutting tools (see, for example, Kobayashi et al U.S. Pat. No. 4,169,913). In the prior art PVD titanium nitride coatings have been commercially applied directly to tool steel and cemented carbide substrates.
European Patent Application Publication No. 0191554 teaches that cemented carbide substrates may be PVD coated with TiC or TiCN if a first PVD TiN coating of between 0.1 and 1 micrometer (.mu.) is applied in order to avoid diffusion between the carbide of the substrate, the titanium carbide in the PVD coating which leads to a reduction in toughness. It is further taught that a PVD TiN layer should be applied over the PVD TiC or TiCN.
It is known that PVD coatings may be applied to cemented carbide and tool steel substrates by a variety of techniques, such as ion plating, magnetic sputtering and arc evaporation. In addition, each technique has many variations. It has been observed that these various techniques and their variations result in PVD coated tools with a variety of properties. Depending on the exact technique used to deposit the coating, properties such as coating hardness, residual stress, tendency to react or bond to the substrate may positively or adversely be affected. These PVD techniques and the properties of the resulting coatings are described in: Buhl et al, "TiN Coatings on Steel,&38 Thin Solid Films, Vol. 80 (1981) pages 265-270; Buhl et al, U.S. Pat. No. 4,448,802 (foregoing described the Balzers AG ion plating technique and equipment used by the applicants herein); Munz et al, "A High Rate Sputtering Process for the Formation of Hard Friction-Reducing TiN Coatings on Tools," Thin Solid Films, Vol. 96 (1982) pages 79-86; Munz et al U.S. Pat. No. 4,426,267; Kamacki et al, "A Comparison of Residual Stresses in Cemented Carbide Cutting Tips Coated with TiN by the CVD and PVD Processes and Their Effect on Failure Resistance," Surfacing Journal International, Vol. 1, No. 3 (1986) pages 82-86; Wolfe et al, "The Role of Hard Coatings in Carbide Milling Tools," Journal of Vacuum Science Technology, A3 (1986) pages 2747-2754; Quinto et al, "High Temperature Microhardness of Hard Coatings Produced by Physical and Chemical Vapor Deposition," Thin Solid Films, Vol. 153 (1987) pages 19-36; Jindal et al, "Adhesion Measurements of Chemically Vapor Deposited and Physically Vapor Deposited Hard Coatings on WC-Co Substrates," Vol. 54 (1987) pages 361-375; Jindal et al, "Load Dependence of Microhardness of Hard Coatings," Surface and Coatings Technology, Vol. 36 (1988) pages 683-694 (Presented at the 15th International Conference on Metallurgical Coatings, San Diego, Calif., U.S.A., Apr. 11-15, 1988); Rickerby et al, "Correlation of Process and System Parameters with Structure and Properties of Physically Vapour-Deposited Hard Coatings," Thin Solid Films, Vol. 157 (February 1988) pages 195-222 (see pages 200, 201 and 219-221); Quinto et al, "Mechanical Properties, Structure and Performance of Chemically Vapor-Deposited and Physically Vapor-Deposited Coated Carbide Tools," Materials Science and Engineering, A105/106 (1988) pages 443-452 (presented at 3rd International Conference on Science of Hard Materials, Nassau, The Bahamas, Nov. 9-13, 1987).
It is the inventors' opinion that the technique that provides the best PVD coating is that described in the Buhl et al patent and article mentioned above which utilizes the Balzers AG ion plating technology and equipment. This belief is based on their analysis of different types of PVD coated tools which have shown that, in PVD TiN coatings, the highest hardnesses and the highest compressive residual stresses are produced in the Balzers AG ion plated (hereinafter BIP) PVD coating. These properties produce a cutting tool that has higher wear resistance and less susceptibility to edge chipping and breakage than possessed by other PVD coated cutting tools.
However, the high residual compressive stress of the BIP PVD coatings have also produced problems. When the present inventors sought to produce cutting tools having a BIP-PVD TiCN coating thereon, the TiCN coatings produced were susceptible to flaking off. Similar results were observed with BIP-PVD TiC coatings. Analysis of the tools indicated that the BIP-PVD TiCN contained very high compressive residual stresses, perhaps twice as high as that found in BIP-PVD TiN, but no brittle diffusion phases such as reported by the aforementioned European Patent Application. It was, therefore, theorized that this was the cause of the flaking problem. This problem was resolved by first applying a BIP-PVD TiN coating to the surface of the substrate and then applying the BIP-PVD TiCN coating. Tool steel and cemented carbide tools having the foregoing BIP-PVD TiN/TiCN two layer coating have since been publicly disclosed.
It was then desired to apply a further BIP-PVD TiN coating over the BIP-PVD TiN/TiCN coatings described above to provide improved wear resistance and appearance. However, it was not known whether the BIP-PVD TiN could be adherently bonded to the existing BIP-PVD TiCN coating due to the high compressive residual stresses in the various coatings.